1. Field of the Invention
The present invention relates to a television tuner, and particularly, to a television tuner capable of improving image disturbance when receiving a UHF band television signal.
2. Description of the Related Art
FIG. 5 shows a structure of a conventional television tuner. In FIG. 5, VHF-band and UHF-band television signals are input to a VHF input tuning circuit 2 and a UHF input tuning circuit 3 through a trap circuit 1 for removing an unnecessary signal such as an FM broadcasting signal.
The VHF input tuning circuit 2 comprises a tuning varactor diode 2a of which anode is connected to a ground, a high-band coil 2b, a low-band coil 2c, a low-band coil 2d and a high-band coil 2e, which are connected in series to a cathode in sequential high-frequency wise and are connected in parallel to the tuning varactor diode 2a in high-frequency wise, and a switch diode 2f connected in parallel to the two low-band coils 2c and 2d in high-frequency wise. The high-band coil 2e at the low potential side is connected to a ground in high-frequency wise through a DC cut capacitor 2g. In addition, the television signal is input to a connection point between the two low-band coils 2c and 2d, and a connection point between the cathode of the tuning varactor diode 2a and the high-band coil 2b serves as an output terminal of the VHF input tuning circuit 2.
In addition, a connection point between the high-band coil 2e and the DC cut capacitor 2g is connected to a ground through a resistor 2h and is connected to a power supply terminal B. Therefore, a bias voltage is applied to the anode of the switch diode 2f. In addition, the cathode of the switch diode 2f is applied with a switching voltage having a high level (5 volt) or a low level (0 volt) from a band switching terminal Sw. The cathode of the tuning varactor diode 2a is applied with a tuning voltage from a tuning voltage terminal Tu.
At a next stage of the VHF input tuning circuit 2, a VHF high frequency amplifier 4 is provided, and the VHF input tuning circuit 2 and the VHF high frequency amplifier 4 are coupled with each other by a coupling varactor diode 5. At a next stage of the VHF high frequency amplifier 4, a VHF interstage tuning circuit 10 is provided.
Furthermore, a capacitor 8 is connected between the connection point of the two low-band coils 2c and 2d and the anode of the coupling varactor diode 5, and a capacitor 9 is connected between the cathode of the switch diode 2f and the anode of the coupling varactor diode 5. The capacitor 8 acts to attenuate the image frequency band, together with the high-band coil 2b, the low-band coil 2c and the coupling varactor diode 5.
The UHF input tuning circuit 3 has a coil 3a whose one end is connected to a ground through a DC cut capacitor 3f and two tuning varactor diodes 3b and 3c. The two tuning varactor diodes 3b and 3c have the same characteristics and cathodes are connected to each other. The anode of one tuning varactor diode 3b is connected to the other end of the coil 3a. A connection point between the coil 3a and the tuning varactor diode 3b is connected to the connection point between the low-band coils 2c and 2d and is coupled with the trap circuit 1. In addition, the anode of the other tuning varactor diode 3c is connected to the connection point between the coil 3a and the DC cut capacitor 3f through a tuning frequency correcting capacitor 3d and a resistor 3e. 
In addition, the cathodes of the two tuning varactor diodes 3b and 3c are connected to the tuning terminal Tu and are connected to a UHF high frequency amplifier 12 through a DC cut capacitor 11. At a next stage of the UHF high frequency amplifier 12, an UHF interstage tuning circuit 13 is provided.
In the above-mentioned structure, in the case of receiving the VHF band television signal, the VHF high frequency amplifier 4 becomes an operation state and the UHF high frequency amplifier 12 becomes a non-operation state. In the case of receiving the UHF band television signal, the UHF high frequency amplifier 12 becomes an operation state and the VHF high frequency amplifier 4 becomes a non-active state.
In addition, in the case of receiving the high band television signal in the VHF band, the switch diode 2f is turned on and the VHF input tuning circuit 2 is tuned to the high band. In the case of receiving the low band television signal in the VHF band, the switch diode 2f is turned off and the VHF input tuning circuit 2 is turned to the low band.
In addition, the tuning frequency is changed by the tuning voltage applied to the tuning varactor diode 2a. 
Further, since in the UHF input tuning circuit 3, a bias voltage V is applied from the VHF input tuning circuit to the anodes of the two tuning varactor diodes 3b and 3c, the range of the tuning frequency becomes widens. In addition, the tracking with a local oscillating frequency is obtained by the tuning frequency correcting capacitor 3d. (for example, see Japanese Unexamined Patent Application Publication No. 2003-134410 (FIG. 1)).
In the above-mentioned structure, since the capacitor for attenuating the image frequency band is provided in the VHF input tuning circuit but is not provided in the UHF input tuning circuit, there is a problem in that the disturbance ratio of the image when receiving the UHF band television signal is lower than or equal to 60 dB.